| → that 1859 1860 |
| (chiefly from the greatly reduced numbers of nests in the spring) that 1861 1866 1869 1872 |
|
|
gradually kill the less vigorous, though fully
thus out of twenty species growing on a little plot of
(three feet by four) nine species
from the other species being allowed to grow up freely. |
|
| The amount of food for each species of course
the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. If not one head of game were shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually
On the other hand, in some cases, as with the
none are destroyed by beasts of
the tiger in India most rarely dares to attack a young elephant protected by its dam. |
|
| Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or
to be the most effective of all checks. I estimated
→that
the winter of
destroyed four-fifths of the birds in my own grounds; and this is a tremendous destruction, when we remember that ten per
is an extraordinarily severe mortality from epidemics with man. The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when climate, for instance extreme cold,
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